Method of manufacturing a window transparent for electrons of an electron beam in particular of an x-ray source

ABSTRACT

The present invention relates to a method of manufacturing a window transparent for electrons of an electron beam (E), in particular of an X-ray source. In order to enable a less costly and elaborate manufacture of such a window and in order to prevent unwanted sharp edges in a window area which may damage the window foil ( 2 ), a method is proposed comprising the steps of:—providing on a surface ( 11 ) of a carrier element ( 1 ) to which a window foil ( 2 ) shall be a fixed a receiving area ( 13, 16 ) for receiving a soldering material ( 3 ) used for fixing said window foil ( 2 ) to said carrier element ( 1 ), said carrier element ( 1 ) comprising a through hole ( 12 ) for the transmission of said electrons (E),—covering said surface ( 11 ) having said receiving area ( 13, 16 ) with a soldering material ( 3 ) such that substantially only said receiving area ( 13, 16 ) is filled with soldering material ( 3 ),—placing said window foil ( 2 ) on top of said surface ( 1 ) and—heating said soldering material ( 3 ) for fixing said window foil ( 2 ) to said surface ( 11 ).

The present invention relates to a method of manufacturing a windowtransparent for electrons of an electron beam, in particular of an X-raysource. Further, the present invention relates to such a window as wellas to an X-ray source including such a window.

An X-ray tube having a liquid metal target is known from U.S. Pat. No.6,185,277 B1. The electrons emitted by an electron source enter theliquid metal through a thin window and produce X-rays therein. Theliquid metal, having a high atomic number, circulates under theinfluence of a pump so that the heat produced by the interaction withthe electrons in the window and the liquid metal can be dissipated. Theheat generated at this area is dissipated by a turbulent flow, thusensuring effective cooling.

The window is constructed in such a manner that on the one hand it is asstable as possible so as to withstand the flow pressure of thecirculating liquid metal, and that on the other hand it should draw aslittle as possible energy from the electrons. A suitable material forthe window is diamond, other materials are, for instance, beryllium,tungsten or a tungsten alloy. During operation the window is subjectedto extreme conditions such as temperatures up to 1000° C. and pressuresup to 10 bar. Further, the window is subjected to a corrosive influenceof the liquid metal.

Such a window is generally manufactured by use of a high temperaturesoldering process. Therein, the thin window foil having a typicalthickness of below 10 μm, for instance 5 μm, is soldered onto a carrierelement, such as a metal frame made of molybdenum, using an activesoldering material. In a first step the soldering material has to beformed in a thickness in the order of magnitude of the window foilthickness. The metal frame has a slit-like opening of a several squaremillimeters size which is covered by the window foil. Metal frame,soldering layer and window foil are then stacked upon each other andfinally soldered at a temperature of approximately 950° C.

A drawback of this manufacturing method is, on the one hand, theelaborate and costly way of manufacturing the solder layer and, on theother hand, the intrusion of excessive soldering material into thewindow area. This means that the electrons are not able at positions,where such soldering material intrudes into the window area, topenetrate through the window foil into the liquid metal, but are alreadyabsorbed in the window area. Furthermore, the intruding solderingmaterial forms an undefined, possibly sharp edge in the window areawhich may be a weak spot as regards the strain of the window.

It is thus an object of the present invention to provide an improvedmethod of manufacturing a window transparent for electrons of anelectron beam, in particular for use in an X-ray source as describedabove, which avoids these drawbacks and is, in particular, less costlyand elaborate. It is a further object of the present invention toprovide a window manufactured according to this method as well as anX-ray source having such a window.

This object is achieved according to the present invention by a methodas claimed in claim 1, comprising the steps of:

providing on a surface of a carrier element to which a window foil shallbe a fixed a receiving area for receiving a soldering material used fora fixing said window foil to said carrier element, said carrier elementcomprising a through hole for the transmission of said electrons,

covering said surface having said receiving area with a solderingmaterial such that substantially only said receiving area is filled withsoldering material,

placing said window foil on top of said surface and

heating said soldering material for fixing said window foil to saidsurface.

The present invention is based on the idea to provide a structuring inone surface of other carrier element prior to soldering of the windowfoil to said carrier element in order to provide a receiving areatherein for receiving the soldering material as a kind of solder depot.This receiving area shall prevent the intrusion of excessive solderingmaterial into the window area, in particular into the through hole ofthe carrier element. After said structuring for providing said receivingarea, the surface with said receiving area is covered with solderingmaterial such that the soldering material is only provided in thereceiving area. Thereupon the window foil is placed and soldered byheating the soldering material which thus flows into the area betweenthe window foil and the carrier element due to capillary forces tosufficiently fix the window foil to the surface of the carrier element.It is thus prevented that excessive soldering material flows into thethrough hole and forms undefined or sharp edges which may damage thewindow foil during operation.

Preferred embodiments of the invention are defined in the dependentclaims. The invention also relates to a window for the transmission ofelectrons of an electron beam, in particular for use in an X-ray source,comprising:

a carrier element for carrying a window foil, said carrier elementcomprising a through hole for the transmission of said electrons,

a window foil soldered on a surface of said carrier element, whereinsaid surface of said carrier element is provided with a receiving areafilled with a soldering material used for a fixing said window foil tosaid carrier element.

Still further, the present invention relates to an X-ray source havingan electron source for emitting an electron beam, a target for emittingX-rays upon incidence of said electron beam and a window as describedabove, said window being located between said electron source and saidtarget. In an embodiment, the X-ray source comprises a liquid metaltarget and is of the type as disclosed in U.S. Pat. No. 6,185,277 B1.

In a preferred embodiment the step of covering said surface having saidreceiving area comprises two sub-steps. In a first sub-step the completesurface is covered with soldering material, i.e. not only the receivingarea. In a second sub-step, excessive soldering material is removed, forinstance-using a milling or grinding process so that essentially onlysaid receiving area is finally filled with soldering material.Preferably, said receiving area comprises one or more grooves aroundsaid through hole in said carrier element. These grooves are, forinstance, made by use of a milling cutter or a laser and can, forinstance, have a depth of 50 μm and a width of 50 μm when said window isto be used in an X-ray source having a liquid metal target. Such groovescan be easily made and efficiently receive soldering material. Thegrooves may have different cross-sections, such as rectangular ortriangular cross-sections. However, semi-circular cross-sections of thegrooves are preferred, since the soldering material can develop free ofpores therein. In corners having sharp edges of sharp or rectangulargrooves bubbles can be formed in the soldering material, which prevent acomplete moistening or covering by the soldering material. The betterthe grooves are filled with soldering material, the more evenly will bethe layer of soldering material in the plane facing the window foil.

In another embodiment the grooves are concentric around said throughhole, i.e. the receiving area comprises a number of grooves having adifferent radius around said through hole. In this embodiment it isavoided that soldering material flows from one groove into anothergroove. However, the grooves may also be provided spirally around saidthrough hole.

According to an alternative embodiment the receiving area is made byablating the surface of said carrier element to which said window foilshall be fixed so as to obtain a carrier element having an inclinedsurface with a height decreasing from said through hole to its edge.Thus, the carrier element has a sloping surface so that there exists awedge-shaped gap between this surface and the plane window foil whenplacing it on top of said surface.

In order to further avoid that the window foil gets damaged at the inneredges of the carrier element, the edge of the surface of the carrierelement facing the through hole is rounded.

In addition to forming grooves or a wedge-shaped gap as receiving area,a channel may be further provided closely surrounding the through holefor preventing soldering material to flow into the through hole. Forinstance, in the embodiment where the receiving area comprises a numberof grooves the inner-most groove may be provided as such a channel,having an increased depth and/or width compared to the other grooves.

In still a further embodiment the carrier element comprises a topcarrier element and a bottom carrier element, said receiving area beingprovided in said top carrier element and said window foil being fixed tosaid top carrier element before said top carrier element being solderedto said bottom carrier element. Preferably, for soldering the windowfoil to the top carrier element a soldering material having a highsoldering temperature is used while for soldering, in a subsequent step,the top carrier element to the bottom carrier element, a solderingmaterial having a lower soldering temperature is used.

By splitting the process into two sub-processes a high number of windowscan be manufactured in one batch. After the first sub-process thenon-optimal windows (top carrier elements) can be easily sorted out, sothat the much more expensive bottom carrier elements are subsequentlyonly fixed to optimum top carrier elements, leading to a cost reduction.

The present invention will now be explained in more detail withreference to the drawings in which

FIG. 1 shows a window manufactured by a known method,

FIG. 2 illustrates the steps of the method according to the presentinvention,

FIG. 3 shows a window manufactured according to the method of theinvention,

FIG. 4 shows a top view on the receiving area,

FIGS. 5 to 10 show different embodiments of a window manufacturedaccording to different embodiments of the method of the invention, and

FIG. 11 shows an X-ray source according to the present invention.

FIG. 1 shows a window manufactured by a known manufacturing method. Itcomprises a metal carrier element 1 and a window foil 2 soldered on thetop surface 11 of the carrier element. For soldering the window foil 2to the carrier element 1 the soldering material 3 is first put on top ofthe surface 11. In an elaborate processing the soldering material 3 hasto be treated to be of the same size in thickness as the window foil 2whereafter the soldering material 3 is heated to a temperature ofapproximately 950° C. to fix the window foil 2 to the carrier element 1.

The central part 22 of the window foil 2 covers a through hole 12provided in the center of the carrier element 1. The window foil 2 thusserves to separate liquid metal, in case of use of the window in anX-ray source having a liquid metal target, from a vacuum area. Duringoperation an electron beam E of electrons transmits the window foil 22in order to enter the liquid metal and produce X-rays therein.

As can be seen in FIG. 1 part of the soldering material 3 is, duringheating of the soldering material, intruded into the through hole 12 andforms an undesired solder portion 31. These avoid electrons frompenetrating into the liquid metal at these areas and thus reduce theefficiency of the X-ray source. Further, these portions 31 may form anundefined edge which may either destroy or damage the window foil 2during operation.

The steps of the manufacturing method according to the present inventionare illustrated in FIG. 2. Therein, a cross-section through only a partof the carrier element or the manufactured window, respectively, isshown. In a first step, illustrated in FIG. 2 a, the carrier element isprovided with a number of concentric grooves 13 formed in the surface 11around the through hole 12 (not shown here). These grooves 13 constitutea so-called receiving area for receiving soldering material in asubsequent step.

In a second step, shown in FIG. 2 b, a soldering material 3 is put ontop of the pre-grooved surface 11. By heating the soldering material 3it becomes fluid and flows, at least partly, into the grooves 13 whichare finally filled with soldering material as shown in FIG. 2 c. Thethickness of the layer of soldering material 3 is primarily not ofimportance since in a subsequent step, as shown in FIG. 2 d, excessivesoldering material 3 which is still on top of the surface 11 but not inthe grooves 13 is removed, for instance by grinding or milling so thatessentially only in the grooves 13 soldering material 3 is present.

Finally, the soldering material 3 is heated without the use of anyadditional soldering material. The soldering material 3 present in thegrooves 13 flows out of the grooves due to capillary forces into thearea between the window 2, which has been put on top of the surface 11,and the surface 11 itself and finally provides a secure fixture betweenthe window foil 2 and the carrier element 1, as shown in FIG. 2 e.

A window according to the present manufactured by the above describedmethod is shown in FIG. 3. Shown are the carrier element 1 with thepre-grooved surface 11 having grooves 13 filled with soldering material3 which has filled the area between the surface 11 and the window foil 2when being heated. As can be seen from FIG. 3, no soldering material 3has come into the through hole 12 so that neither the efficiency isreduced nor undesired and sharp edges are formed which may damage ordestroy the window foil 2. In order to further prevent any damage of thewindow foil the edges 14 of the surface 11 facing the through hole 12are rounded, i.e. comprise a curved radius.

A top view on the surface 11 of the window shown in FIG. 3 is depictedin FIG. 4. Therein the through hole 12 which has the form of anelongated hole is surrounded by a number of grooves which areapproximately concentric around the through hole 12. Such grooves 13can, for instance, easily be made by use of a milling cutter or a laser.

FIGS. 5 to 10 show, as a cross-section, further embodiments of a windowaccording to the present invention at which different modifications ofthe manufacturing method and the window are illustrated. For instance,as shown in FIG. 5, the grooves 13 have a semi-circular cross-sectionwhile in the embodiment shown in FIG. 2, the grooves 13 have arectangular cross-section.

In the embodiment shown in FIG. 6 an additional channel 15 is providedas inner-most groove having an increased width and depth compared to thegrooves 13. This additional channel 15 serves as stopping groove forstopping any soldering material 3 flowing in the direction of thethrough hole 12 in order to prevent that the soldering material flowsinto the through hole 12.

In the embodiment shown in FIG. 7 the carrier element 1 is divided intoa top carrier element 101 and a bottom carrier element 102. While atfirst the grooves 13 are provided in the top carrier element 101 and thewindow foil 2 is soldered to the top carrier element 101 as describedabove, in a subsequent step the top carrier element 101 is soldered tothe bottom carrier element 102 in which process soldering material 4 canbe used having a lower melting temperature than the soldering material3.

The embodiment shown in FIG. 8 mainly corresponds to the embodimentshown in FIG. 7 but includes an additional channel 15 as already shownand explained with reference to FIG. 6.

A different embodiment of a receiving area is shown in FIG. 9 where thecarrier element 1 is also sub-divided into a top carrier element 101 anda bottom carrier element 102 soldered together by a soldering material4. However, the top surface 11 of the top carrier element 101 does nothave grooves as shown in previous figures but is inclined so as to forma wedge-shaped gap 16 between the window foil 2 and the surface 11. Saidgap 16 has an increasing thickness from the side adjacent the throughhold 12 to the outer edge and is provided for receiving the solderingmaterial to prevents, due to the small thickness adjacent the throughhole 12, that soldering material can flow into the through hole 12.

The embodiment shown in FIG. 10 corresponds to the embodiment shown inFIG. 9, but includes an additional channel 15 as already shown andexplained with reference to FIG. 6.

An embodiment of an X-ray source according to the present invention inwhich such a window is preferably used, is shown in FIG. 11. Itcomprises a preferably electrically grounded tube envelope 40 which issealed in a vacuum tight manner by the window 41. In the vacuum space ofthe tube envelope 40 there is accommodated an electron source in theform of a cathode 42 which emits an electron beam E in the operatingcondition, which electron beam E is incident, through the window 41, ona liquid metal present in a target system 44. The target system 44includes a system of ducts 440 in which the liquid metal is driven by apump 442 and flows past the outer side of the window 41 in a section441. After having past the section 441, it enters a heat exchanger 443wherefrom the heat produced can be drained by means of a suitablecooling circuit. The interaction between the electrons passing throughthe window 41 and the liquid metal produces X-rays, i.e. the liquidmetal serves as a target, which emanate through the window 41 and anX-ray window 45 in the envelope 40. For further details of the X-raysource reference is made to the above mentioned U.S. Pat. No. 6,185,277B1 which is herein incorporated by reference.

1. Method of manufacturing a window transparent for electrons of anelectron beam (E), in particular of an X-ray source, comprising thesteps of: providing on a surface (11) of a carrier element (1) to whicha window foil (2) shall be a fixed a receiving area (13, 16) forreceiving a soldering material (3) used for fixing said window foil (2)to said carrier element (1), said carrier element (1) comprising athrough hole (12) for the transmission of said electrons (E), coveringsaid surface (11) having said receiving area (12) with a solderingmaterial (3) such that substantially only said receiving area (13, 16)is filled with soldering material (3), placing said window foil (2) ontop of said surface (11) and heating said soldering material (3) forfixing said window foil (2) to said surface (11).
 2. Method as claimedin claim 1, wherein in said step of covering said surface (11) with saidsoldering material (3) excessive soldering material is removed so thatessentially only said receiving area (13, 16) is filled with solderingmaterial (3).
 3. Method as claimed in claim 1, wherein said receivingarea comprises one or more grooves (13) around said through hole (12) insaid carrier element (1).
 4. Method as claimed in claim 3, wherein saidgrooves (13) have a semi-circular cross-section.
 5. Method as claimed inclaim 3, wherein said grooves (13) are concentric around said throughhole (12).
 6. Method as claimed in claim 1, wherein said receiving areais made by ablating said surface of said carrier element so as to obtaina carrier element (1) having an inclined surface (16) with a heightdecreasing from said through hole (12) to its edge.
 7. Method as claimedin claim 1, wherein the edge (13) of the surface (1) of said carrierelement (1) facing said through hole (12) is rounded.
 8. Method asclaimed in claim 1, wherein said receiving area comprises a channel (15)closely surrounding said through hole (12) for preventing solderingmaterial (3) to flow into said through hole (12).
 9. Method as claimedin claim 1, wherein said carrier element (1) comprises a top carrierelement (101) and a bottom carrier element (102), said receiving area(13, 16) being provided in said top carrier element (101) and saidwindow foil (2) being fixed to said top carrier element (101) beforesaid top carrier element (101) being soldered to said bottom carrierelement (102).
 10. Window for the transmission of electrons of anelectron beam (E), in particular of an X-ray source, comprising: acarrier element (1) for carrying a window foil (2), said carrier element(1) comprising a through hole (12) for the transmission of saidelectrons (E), a window foil (2) soldered on a surface (11) of saidcarrier element (1), wherein said surface (11) of said carrier element(1) is provided with a receiving area (13, 16) filled with a solderingmaterial (3) used for a fixing said window foil (2) to said carrierelement (1).
 11. X-ray source having an electron source (42) foremitting an electron beam (E), a target (44) for emitting X-rays uponincidence of said electron beam (E) and a window (41) as claimed inclaim 10, said window (41) being located between said electron source(42) and said target (44).
 12. X-ray source as claimed in claim 11,wherein said target (44) is a liquid metal target and wherein saidwindow (41) separates the liquid metal of said liquid metal target (44)from a vacuum area (40) comprising said electron source (42).